Voltage-gated sodium channels (VGSCs) are responsible for the action potential in nerve and muscle. Drugs (such as local anesthetics) and toxins (such as tetrodotoxin, TTX) that block VGSCs have been mainstays in pain medication and basic neuroscience research. With notable exceptions, these small molecules do not discriminate readily among the various subtypes of VGSCs. For example, TTX is a very potent blocker of both the skeletal muscle subtype (NaV1.4) and several of the subtypes found in brain (e.g., NaV1.2). In contrast, the classic mu-conopeptides readily block NaV1.4 but not NaV1.2. A simple explanation for the greater specificity of conopeptides is that they are much larger than TTX with a correspondingly larger "footprint";i.e., the larger ligand has more locations on its surface that must complement the surface on its cognate channel to which it binds. The major thrust of the proposed research is to generate conopeptides that target specific subtypes of VGSCs, particularly those (including TTX-resistant ones) that are found in sensory neurons that convey pain information. Thus far, we have discovered new mu-conopeptides that: a) irreversibly block a fraction of the TTX-resistant sodium currents in sensory neurons of rodents;b) discriminate among the TTX-sensitive subtypes with a specificity different from that of classic mu-conopeptides (e.g., block NaV1.2 more effectively than NaV1.4);and c) are analgesic in rodents. We also found that a member of a different family of conopeptides, muO-conopeptide MrVIB, blocks TTX-resistant (in addition to TTX-sensitive) VGSCs and also exhibits analgesic activity in rats. The two conopeptides block VGSCs differently: mu-conopeptides essentially plug the pore of the channel, whereas muO-conopeptides act by a mechanism that remains to be established. We will scrutinize these conopeptides and their synthetic derivatives to: a) obtain ligands for VGSCs that are more selective, and b) elucidate their mechanisms of action. Conopeptide activities will be scrutinized mainly by electrophysiology of mammalian primary sensory neurons and Xenopus oocytes expressing cloned mammalian VGSCs. These conopeptides are potential analgesics and lead compounds that may be developed into medications to alleviate pain.